Stereolithographic method and apparatus with enhanced control of prescribed stimulation production and application

ABSTRACT

A rapid prototyping and manufacturing (e.g. stereolithography) method and apparatus for producing three-dimensional objects by selectively subjecting a liquid or other fluid-like material to a beam of prescribed stimulation. In a preferred embodiment a source of prescribed stimulation is controlled to reduce or inhibit the production of the prescribed stimulation during at least some periods when the prescribed stimulation is not needed to expose the material. In another preferred embodiment, the source of stimulation is controlled to vary the quantity of prescribed stimulation that is produced and allowed to reach the material. In an additional preferred embodiment control of laser output occurs based on a combination of supplying a regulated amount of voltage to an AOM in conjunction with temporary sensing of laser power and a known desired power to attain. In a further preferred embodiment, a quantity of prescribed stimulation may be set by consideration of desired solidification depths to be used, beam profile characteristics, material properties, and scanning speed limitations for different data types. In a still further preferred embodiment, a transition between selected consecutive exposure vectors is performed by scanning one or more interposed non-exposure vectors with the beam inhibited from reaching the building material.

FIELD OF THE INVENTION

This invention relates to the formation of three-dimensional objectsusing a Rapid Prototyping and Manufacturing (RP&M) technique (e.g.stereolithography). The invention more particularly relates to theformation of three-dimensional objects using enhanced production controlof prescribed stimulation and its application to a building material.

BACKGROUND OF THE INVENTION

1. Related Art

Rapid Prototyping and Manufacturing (RP&M) is the name given to a fieldof technologies that can be used to form three-dimensional objectsrapidly and automatically from three-dimensional computer datarepresenting the objects. RP&M can be considered to include threeclasses of technologies: (1) Stereolithography, (2) Selective DepositionModeling, and (3) Laminated Object Manufacturing.

The stereolithography class of technologies create three-dimensionalobjects based on the successive formation of layers of a fluid-likematerial adjacent to previously formed layers of material and theselective solidification of those layers according to cross-sectionaldata representing successive slices of the three-dimensional object inorder to form and adhere laminae (i.e. solidified layers). One specificstereolithography technology is known simply as stereolithography anduses a liquid material that is selectively solidified by exposing it toprescribed stimulation. The liquid material is typically a photopolymerand the prescribed stimulation is typically visible or ultravioletelectromagnetic radiation. The radiation is typically produced by alaser though other sources of radiation are possible such as arc lamps,resistive lamps, and the like. Exposure may occur by scanning a beam orby controlling a flood exposure by use of a light valve that selectivelytransmits or reflects the radiation. Liquid-based stereolithography isdisclosed in various patents, applications, and publications of which anumber are briefly described in the Related Applications sectionhereafter.

Another stereolithography technology is known as Selective LaserSintering (SLS). SLS is based on the selective solidification of layersof a powdered material by exposing the layers to infraredelectromagnetic radiation to sinter or fuse the powder particles. SLS isdescribed in U.S. Pat. No. 4,863,538, issued Sep. 5, 1989, to Deckard. Athird technology is known as Three Dimensional Printing (3DP). 3DP isbased on the selective solidification of layers of a powdered materialwhich are solidified by the selective deposition of a binder thereon.3DP is described in U.S. Pat. No. 5,204,055, issued Apr. 20, 1993, toSachs.

The present invention is primarily directed to stereolithography usingliquid-based building materials (i.e. medium). It is believed, however,that the techniques of the present invention may have application in theother stereolithography technologies.

Selective Deposition Modeling, SDM, involves the build-up ofthree-dimensional objects by selectively depositing solidifiablematerial on a lamina-by-lamina basis according to cross-sectional datarepresenting slices of the three-dimensional object. The material beingdispensed may be solidified upon cooling, by heating, exposing toradiation, or upon application of a second physical material. A singlematerial may be dispensed or multiple materials dispensed with eachhaving different properties. One such technique is called FusedDeposition Modeling, FDM, and involves the extrusion of streams ofheated, flowable material which solidify as they are dispensed onto thepreviously formed laminae of the object. FDM is described in U.S. Pat.No. 5,121,329, issued Jun. 9, 1992, to Crump. Another technique iscalled Ballistic Particle Manufacturing, BPM, which uses a 5-axis,ink-jet dispenser to direct particles of a material onto previouslysolidified layers of the object. BPM is described in PCT publicationnumbers WO 96-12607, published May 2, 1996, by Brown; WO 96-12608,published May 2, 1996, by Brown; WO 96-12609, published May 2, 1996, byMenhennett; and WO 96-12610, published May 2, 1996, by Menhennett. Athird technique is called Multijet Modeling, MJM, and involves theselective deposition of droplets of material from multiple ink jetorifices to speed the building process. MJM is described in U.S. Pat.No. 5,943,235, filed Sep. 27, 1996, and issued Aug. 24, 1999 to Earl etal. and in U.S. Application Ser. No. 08/722,335, filed Sep. 27, 1996, byLeyden et al. now abandoned (both assigned to 3D Systems, Inc. as is theinstant application).

Laminated Object Manufacturing, LOM, techniques involve the formation ofthree-dimensional objects by the stacking, adhering, and selectivecutting of sheets of material, in a selected order, according to thecross-sectional data representing the three-dimensional object to beformed. LOM is described in U.S. Pat. No. 4,752,352, issued Jun. 21,1988, to Feygin, U.S. Pat. No. 5,015,312, issued May 14, 1991, toKinzie, and U.S. Pat. No. 5,192,559, issued Mar. 9, 1993, to Hull etal.; and in PCT Publication No. WO 95-18009, published Jul. 6, 1995, byMorita.

Though, as noted above, the techniques of the instant invention aredirected primarily to liquid-based stereolithography object formation,it is believed that some of the techniques may have application in theLOM and/or SDM technologies where application of a beam or other laminaeforming element must be precisely controlled.

Needs exist in the stereolithographic arts for improved beam generationtechniques and positioning techniques. A first need exists for enhancedeffective life of solid state ultraviolet producing lasers in astereolithographic system. A second need exists for maintainingsubstantially uniform exposure over the length of each vector whilesimultaneously scanning as fast as possible, maintaining adequatepositional control and minimizing the overall exposure time. A thirdneed exists for improved control of the laser power that is produced andapplied to the building material in a stereolithographic system. Afourth need exists for improved efficiency in exposing the buildingmaterial in a stereolithographic system when exposure is controlled by aplurality of different vector types. A fifth need exists for simplifiedtechniques for determining the maximum useful laser power for exposing agiven set of vectors.

2. Other Related Patents and Applications

The patents, applications, and publications mentioned above andhereafter are all incorporated by reference herein as if set forth infull. Table 1 provides a listing of patents and applications co-owned bythe assignee of the instant application. A brief description of subjectmatter found in each patent and application is included in the table toaid the reader in finding specific types of teachings. It is notintended that the incorporation of subject matter be limited to thosetopics specifically indicated, but instead the incorporation is toinclude all subject matter found in these applications and patents. Theteachings in these incorporated references can be combined with theteachings of the instant application in many ways. For example, thereferences directed to various data manipulation techniques may becombined with the teachings herein to derive even more useful, modifiedobject data that can be used to more accurately and/or efficiently formobjects. As another example, the various apparatus configurationsdisclosed in these references may be used in conjunction with the novelfeatures of the instant invention.

                  TABLE 1                                                         ______________________________________                                        Related Patents and Applications                                              Pat. No.                                                                      Issue Date                                                                    Application                                                                   No.                                                                           Filing Date                                                                            Inventor   Subject                                                   ______________________________________                                        4,575,330                                                                              Hull       Discloses fundamental elements of                         Mar 11, 1986        stereolithography.                                        06/638,905                                                                    Aug 8, 1984                                                                   4,999,143                                                                              Hull, et al.                                                                             Discloses various removable support                       Mar 12, 1991        structures applicable to                                  07/182,801          stereolithography.                                        Apr 18, 1988                                                                  5,058,988                                                                              Spence     Discloses the application of beam                         Oct 22, 1991        profiling techniques useful                               07/268,816          in stereolithography for determining                      Nov 8, 1988         cure depth and scanning velocity, etc.                    5,059,021                                                                              Spence, et al.                                                                           Discloses the utilization of drift                        Oct 22, 1991        correction techniques for eliminating                     07/268,907          errors in beam positioning resulting                      Nov 8, 1988         from instabilities in the beam                                                scanning system                                           5,076,974                                                                              Modrek, et al.                                                                           Discloses techniques for post processing                  Dec 31, 1991        objects formed by stereolithography.                      07/268,429          In particular exposure techniques are                     Nov 8, 1988         described that complete the solidifi-                                         cation of the building material. Other                                        post processing steps are also disclosed                                      such as steps of filling in or sanding                                        off surface discontinuities.                              5,104,592                                                                              Hull       Discloses various techniques for reduc-                   Apr 14, 1992        ing distortion, and particularly curl type                07/339,246          distortion, in objects being formed by                    Apr 17, 1989        stereolithography.                                        5,123,734                                                                              Spence, et al.                                                                           Discloses techniques for calibrating a                    Jun 23, 1992        scanning system. In particular                            07/268,837          techniques for mapping from rotational                    Nov 8, 1988         mirror coordinates to planar target                                           surface coordinates are disclosed                         5,133,987                                                                              Spence, et al.                                                                           Discloses the use of a stationary mirror                  Jul 28, 1992        located on an optical path between the                    07/427,885          scanning mirrors and the target surface                   Oct 27, 1989        to fold the optical path in a                                                 stereolithography system.                                 5,141,680                                                                              Almquist, et al.                                                                         Discloses various techniques for                          Aug 25, 1992        selectively dispensing a material to                      07/592,559          build up three-dimensional objects.                       Oct 4, 1990                                                                   5,143,663                                                                              Leyden, et al.                                                                           Discloses a combined stereolithography                    Sep 1, 1992         system for building and cleaning                          07/365,444          objects.                                                  Jun 12, 1989                                                                  5,174,931                                                                              Almquist, et al.                                                                         Discloses various doctor blade                            Dec 29, 1992        configurations for use in forming                         07/515,479          coatings of medium adjacent to                            Apr 27, 1990        previously solidified laminae.                            5,182,056                                                                              Spence, et al.                                                                           Discloses the use of multiple wave-                       Jan 26, 1993        lengths in the exposure of a                              07/429,911          stereolithographic medium.                                Oct 27, 1989                                                                  5,182,715                                                                              Vorgitch, et al.                                                                         Discloses various elements of a large                     Jan 26, 1993        stereolithographic system.                                07/824,819                                                                    Jan 22, 1992                                                                  5,184,307                                                                              Hull, et al.                                                                             Discloses a program called Slice and                      Feb 2, 1993         various techniques for converting                         07/331,644          three-dimensional object data into data                   Mar 31, 1989        descriptive of cross-sections. Disclosed                                      techniques include line width                                                 compensation techniques (erosion                                              routines), and object sizing techniques.                                      The application giving rise to this                                           patent included a number of appendices                                        that provide further details regarding                                        stereolithography methods and systems.                    5,192,469                                                                              Hull, et al.                                                                             Discloses various techniques for                          Mar 9, 1993         forming three-dimensional object from                     07/606,802          sheet material by selectively cutting                     Oct 30, 1990        out and adhering laminae.                                 5,209,878                                                                              Smalley, et al.                                                                          Discloses various techniques for reduc-                   May 11, 1993        ing surface discontinuities between                       07/605,979          successive cross-sections resulting                       Oct 30, 1990        from a layer-by-layer building                                                technique. Disclosed techniques include                                       use of fill layers and meniscus                                               smoothing.                                                5,234,636                                                                              Hull, et al.                                                                             Discloses techniques for reducing                         Aug 10, 1993        surface discontinuities by coating a                      07/929,463          formed object with a material, heating                    Aug 13, 1992        the material to cause it to become                                            flowable, and allowing surface tension                                        to smooth the coating over the object                                         surface.                                                  5,238,639                                                                              Vinson, et al.                                                                           Discloses a technique for minimizing                      Aug 24, 1993        curl distortion by balancing upward                       07/939,549          curl to downward curl.                                    Mar 31, 1992                                                                  5,256,340                                                                              Allison, et al.                                                                          Discloses various build/exposure styles                   Oct 26, 1993        for forming objects including various                     07/906,207          techniques for reducing object                            Jun 25, 1992        distortion. Disclosed techniques include:                 And                 (1) building hollow, partially hollow,                    5,965,079           and solid objects, (2) achieving more                     Oct 12, 1999        uniform cure depth, (3) exposing layers                   08/766,956          as a series of separated tiles or bullets,                Dec 16, 1996        (4) using alternate sequencing exposure                                       patterns from layer to layer, (5) using                                       staggered or offset vectors from layer                                        to layer, and (6) using one or more                                           overlapping exposure patterns per layer.                  5,321,622                                                                              Snead, et al.                                                                            Discloses a computer program known as                     Jun 14, 1994        CSlice which is used to convert                           07/606,191          three-dimensional object data into                        Oct 30, 1990        cross-sectional data. Disclosed                                               techniques include the use of various                                         Boolean operations in stereolithography.                  5,597,520                                                                              Smalley, et al.                                                                          Discloses various exposure techniques                     Jan 28, 1997        for enhancing object formation                            08/233,027          accuracy. Disclosed techniques address                    Apr 25, 1994        formation of high resolution objects                      And                 from building materials that have a                       5,999,184           Minimum Solidification Depth greater                      Dec 7, 1999         than one layer thickness and/or a                         08/428,951          Minimum Recoating Depth greater than                      Apr 25, 1995        the desired object resolution.                            08/722,335                                                                             Thayer, et al.                                                                           Discloses build and support styles for                    Sep 27, 1996        use in a Multi-Jet Modeling selective                     Now                 deposition modeling system.                               abandoned                                                                     5,943,235                                                                              Earl, et al.                                                                             Discloses data manipulation and system                    Aug 24, 1999        control techniques for use in a                           08/722,326          Multi-Jet Modeling selective deposition                   Sep 27, 1996        modeling system.                                          5,902,537                                                                              Almquist, et al.                                                                         Discloses various recoating techniques                    May 11, 1999        for use in stereolithography. Disclosed                   08/790,005          techniques include 1) an inkjet                           Jan 28, 1997        dispensing device, 2) a fling recoater,                                       3) a vacuum applicator, 4) a stream                                           recoater, 5) a counter rotating roller                                        recoater, and 6) a technique for deriving                                     sweep extents.                                            5,840,239                                                                              Partanen, et al.                                                                         Discloses the application of solid-state                  Nov 24, 1998        lasers to stereolithography. Discloses                    08/792,347          the use of a pulsed radiation source                      Jan 31, 1997        for solidifying layers of building                                            material and in particular the ability                                        to limit pulse firing locations to only                                       selected target locations on a surface                                        of the medium.                                            6,001,297                                                                              Partanen, et al.                                                                         Discloses the stereolithographic                          Dec 14, 1999        formation of objects using a pulsed                       08/847,855          radiation source where pulsing occurs                     Apr 28, 1997        at selected positions on the surface                                          of a building material.                                   08/855,125                                                                             Nguyen, et al.                                                                           Discloses techniques for interpolating                    May 13, 1997        originally supplied cross-sectional                                           data descriptive of a three-dimensional                                       object to produce modified data                                               descriptive of the three-dimensional                                          object including data descriptive of                                          intermediate regions between the                                              originally supplied cross-sections                                            of data.                                                  5,945,058                                                                              Manners, et al.                                                                          Discloses techniques for identifying                      Aug 31, 1999        features of partially formed objects.                     08/854,950          Identifiable features include trapped                     May 13, 1997        volumes, effective trapped volumes,                                           and solid features of a specified size.                                       The identified regions can be used in                                         automatically specifying recoating                                            parameters and or exposure parameters.                    5,902,538                                                                              Kruger, et al.                                                                           Discloses simplified techniques for                       May 11, 1999        making high-resolution objects utilizing                  08/920,428          low-resolution materials that are limited                 Aug 29, 1997        by their inability to reliably form                                           coatings of a desired thickness due                                           to a Minimum Recoating Depth (MRD)                                            limitation. Data manipulation                                                 techniques define layers as primary or                                        secondary. Recoating techniques are                                           described which can be used when the                                          thickness between consecutive layers                                          is less than a leading edge bulge                                             phenomena.                                                09/061,796                                                                             Wu, et al. Discloses use of frequency converted                      Apr 16, 1998        solid state lasers in stereolithography.                  09/154,967                                                                             Nguyen, et al.                                                                           Discloses techniques for stereolitho-                     Sep 17, 1998        graphic recoating using a sweeping                                            recoating device that pause and/or slows                                      down over laminae that are being                                              coated over.                                              09/484,984                                                                             Earl, et al.                                                                             Entitled "Method and Apparatus for                        Jan 18, 2000        Forming Three-Dimensional Objects                                             Using Line Width Compensation with                                            Small Feature Retention." Discloses                                           techniques for forming objects while                                          compensating for solidification width                                         induced in a building material by a                                           beam of prescribed stimulation.                           09/246,504                                                                             Guertin, et al.                                                                          Entitled "Method and Apparatus for                        Feb 8, 1999         Stereolithographically Forming Three                                          Dimensional Objects With Reduced                                              Distortion." Discloses techniques                                             for forming objects wherein a delay is                                        made to occur between successive                                              exposures of a selected region of                                             a layer.                                                  09/248,352                                                                             Manners, et al.                                                                          Entitled Stereolithographic Method and                    Feb 8, 1999         Apparatus for Production of Three                                             Dimensional Object Using Multiple                                             Beams of Different Diameters"                                                 Discloses stereolithographic                                                  techniques for forming objects using                                          multiple sized beams including data                                           manipulation techniques for determining                                       which portions of lamina may be                                               formed with a larger beam and which                                           should be formed using a smaller beam.                    09/248,351                                                                             Nguyen, et al.                                                                           Entitled "Stereolithographic Method                       Feb 8, 1999         and Apparatus for Production of Three                                         Dimensional Objects Using Recoating                                           Parameters for Groups of Layers."                                             Discloses improved techniques for                                             managing recoating parameters when                                            forming objects using layer thicknesses                                       smaller than a minimum recoating depth                                        (MRD) and treating some non-consecu-                                          tive layers as primary layers and treat-                                      ing intermediate layers there between as                                      secondary layers.                                         09/246,416                                                                             Bishop, et al.                                                                           Entitled "Rapid Prototyping Apparatus                     Feb 8, 1999         with Enhanced Thermal and Vibrational                                         Stability for Production of Three                                             Dimensional Objects." Discloses an                                            improved Stereolithographic apparatus                                         structure for isolating vibration and/or                                      extraneous heat producing components                                          from other thermal and vibration                                              sensitive components.                                     09/247,113                                                                             Chari, et al.                                                                            Entitled "Stereolithographic Method and                   Feb 8, 1999         Apparatus for production of Three                                             Dimensional Objects with Enhanced                                             thermal Control of the Build                                                  environment. Discloses improved                                               stereolithographic techniques for                                             maintaining build chamber temperature                                         at a desired level. The techniques                                            include varying heat production based                                         on the difference between a detected                                          temperature and the desired                                                   temperature.                                              09/247,119                                                                             Kulkarni, et al.                                                                         Entitled "Stereolithographic Method                       Feb 8, 1999         and Apparatus for Production of Three                                         Dimensional Objects Including                                                 Simplified Build Preparation."                                                Discloses techniques for forming                                              objects using a simplified data                                               preparation process. Selection of the                                         various parameter styles needed to form                                       an object is reduced to answering                                             several questions from lists of                                               possible choices.                                         ______________________________________                                    

The following two books are also incorporated by reference herein as ifset forth in full: (1) Rapid Prototyping and Manufacturing: Fundamentalsof Stereolithography, by Paul F. Jacobs; published by the Society ofManufacturing Engineers, Dearborn Mich.; 1992; and (2) Stereolithographyand other RP&M Technologies: from Rapid Prototyping to Rapid Tooling; byPaul F. Jacobs; published by the Society of Manufacturing Engineers,Dearborn Mich.; 1996.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the quality of vectorscanning in a stereolithography system.

It is a first aspect of the invention to provide a stereolithographicmethod of forming a three-dimensional object from a plurality of adheredlaminae by exposing successive layers of a material to a beam ofprescribed stimulation, including: (1) providing a source of a beam ofprescribed stimulation; (2) forming a layer of material adjacent to anylast formed layer of material in preparation for forming a subsequentlamina of the object; (3) exposing the material to the beam ofprescribed stimulation to form the subsequent lamina of the objectaccording to a plurality of exposure vectors representing the subsequentlamina; and (4) repeating the acts of forming and exposing a pluralityof times in order to form the object from a plurality of adheredlaminae. Providing a plurality of non-exposure vectors between at leastsome pairs of successive exposure vectors, wherein the non-exposurevectors comprise a ramp vector and a jump vector.

It is a second aspect of the invention to provide an stereolithographicapparatus for forming a three-dimensional object from a plurality ofadhered laminae by exposing successive layers of a material to a beam ofprescribed stimulation, including: (1) a source of a beam of prescribedstimulation; (2) a recoating system to form a layer of material adjacentto any last formed layer of material in preparation for forming asubsequent lamina of the object; (3) a scanning system to expose thematerial to the beam of prescribed stimulation to form the subsequentlamina of the object according to a plurality of exposure vectorsrepresenting the subsequent lamina; (4) and a computer programmed tooperate the recoating system and the scanning system to form the objectfrom a plurality of adhered laminae. Software is programmed or ahardware is configured to provide a plurality of non-exposure vectorsbetween at least some pairs of successive exposure vectors, wherein thenon-exposure vectors comprise a ramp vector and a jump vector.

Other aspects of the invention supply apparatus for implementing themethod aspects of the invention noted above.

Additional objects and aspects of the invention will be clear from theembodiments of the invention described below in conjunction with theFigures associated therewith.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1a and 1b depict side views of a stereolithography apparatus forpracticing the instant invention.

FIG. 1c depicts a block diagram illustrating some major components ofthe stereolithography system.

FIG. 1d depicts a schematic diagram illustrating the major components inthe laser head and the beam path followed through the laser head.

FIG. 2a depicts a side view of an object to be formed usingstereolithography

FIG. 2b depicts a side view of the object of FIG. 2a formed usingstereolithography.

FIG. 2c depicts a side view of the object of FIG. 2b where the differentexposure regions associated with each layer are depicted.

FIG. 3 depicts a flow chart of the process of a preferred embodiment.

FIG. 4 depicts a plot of scanning speed for different vector types, IRand UV powers produced by the laser generator over severalrepresentative build stages as used in a preferred embodiment.

FIG. 5 depicts a flow chart of a preferred embodiment.

FIG. 6 depicts a group of hypothetical vectors that are to be used inexposing a layer of material.

FIG. 7 depicts two of the vectors from FIG. 4 along with a number ofnon-exposure vectors that are used in a preferred embodiment totransition between the two exposure vectors.

FIG. 8 depicts a plot of UV and IR power produced by the laser generatorduring tracing of the vectors shown in FIG. 8.

FIG. 9 depicts a flow chart of a preferred embodiment.

FIG. 10 depicts a flow chart of a preferred embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 1a and 1b depict schematic representations of a preferredstereolithography apparatus 1 (SLA) for use with the instant invention.The basic components of an SLA are described in U.S. Pat. Nos.4,575,330; 5,184,307; and 5,182,715 as referenced above. The preferredSLA includes container 3 for holding building material 5 (e.g.photopolymer) from which object 15 will be formed, elevator 7 anddriving means (not shown), elevator platform 9, exposure system 11,recoating bar 13 and driving means (not shown), at least one computer(not shown) for manipulating object data (as needed) and for controllingthe exposure system, elevator, and recoating device.

FIG. 1a depicts the partially formed object as having its most recentlyformed lamina lowered to a position approximately one layer thicknessbelow the desired level of the upper surface of the building material 5(i.e. desired working surface). As the layer thickness is small and thebuilding material very viscous, FIG. 1 a indicates that the material hasnot flowed significantly across the last formed lamina even afterlowering the platform 9. FIG. 1b depicts the coating bar 13 as beingswept part way across the previously formed lamina and that the nextlayer of building material has been partially formed.

A preferred exposure system is described in several of the patents andone application referenced above including U.S. Pat. Nos. 5,058,988;5,059,021; 5,123,734; 5,133,987; 5,840,239; and Ser. No. 09/247,120.This preferred system includes a laser, a beam focusing system, and apair of computer controlled XY rotatable scanning mirrors of either themotor driven or galvanometer type.

FIG. 1c provides a block diagram of selected elements of a preferredstereolithography system 1 wherein like elements are identified withlike numerals. The exposure system includes an IR laser head 70, thatproduces a pulsed beam of radiation operating a desired repetition pulserepetition rate (e.g. 22.5-40 KHz). The exposure system furtherincludes, an AOM 72, a first frequency conversion crystal 74, a secondfrequency conversion crystal 76, two folding mirrors 78, focusing optics80, a pair of XY scanning mirrors 82, and a detector 84. A controlcomputer 86 is provided to preferably control, among other things, thescanning mirrors 82, the AOM 72, the detector 84, and the focusingoptics 80. The optical path is depicted with reference numeral 86. Thecomputer preferably controls the above noted components based on objectdata that has been modified for stereolithographic formation. It ispreferred that the focusing optics be controlled to produce two or morebeam diameters for forming object laminae. The AOM is preferablycontrolled to adjust beam power base on a plurality of criteriaincluding beam size.

The scanning mirrors are used to selectively direct the beam path todesired locations onto the surface of the building material 5 or ontoother items such as detector 84. The optical path beyond the scanningmirrors is depicted with reference numerals 86', 86", or 86'" asexamples of the different directions in which the beam may be directed.The AOM is used to set the beam power that is allowed to proceed fromthe IR laser head 70 to the first and second frequency conversioncrystals. The beam that is allowed to proceed to the frequencyconversion crystals is sent along a first order beam path from the AOM.The other beam path orders (e.g. 0^(th) and 2^(nd)) are inhibited fromprogressing to the frequency conversion crystals. The focusing opticsare used to obtain a desired focus and/or beam diameter at the surface20 of the building material

A more detailed depiction of the beam-generating portion of the exposuresystem is depicted in FIG. 1d wherein like numerals to those used in theother figures depict similar components. The radiation-generatingportion of the exposure system comprises a laser head 68, IR generatinglaser diodes 71, and a fiber optic cable 69. The laser diodes produceapproximately 808 nm radiation at approximately 18 watts. The fiberoptic cable directs the output of the laser diodes 71 to an IR laser 70inside the UV laser head, the radiation from the fiber optic is used tosupply pumping radiation to the IR laser 70. The laser 70 produces 1.064micron radiation that is directed to acousto-optic modulator (AOM) 72that is used to control the beam power by deflecting varying amounts ofthe beam power along different optical paths. A zeroth order opticalpath directs the beam into a beam dump. For example, a trap formed bytwo triangular shaped elements 73. A first order optical path directsthe beam through a half-wave plate 75 that rotates the polarization ofthe beam.

From the half wave plate 75 the beam enters a frequency conversionmodule 93 through an aperture 77. From aperture 77 the beam proceeds tofocusing mirror 79'.

From mirror 79' the beam proceeds through a first frequency conversioncrystal 74. This first crystal 74 converts a portion of the first beaminto a beam that has double the frequency. The remaining portion of theoriginal beam and the beam of doubled frequency proceed to secondfocusing mirror 79", then a third focusing mirror 79'", and then througha second frequency conversion crystal 76. The second crystal 76generates a third beam of tripled frequency compared to the originalbeam that entered first crystal 74. A beam containing all threefrequencies then proceeds out of the conversion module 93 throughaperture 77. The mirrors 78 and other optical elements are wavelengthselective and cause the remaining portions of the original and doubledfrequency beams to attenuate. As such, only the tripled frequencyportion of the beam proceeds along the rest of the beam path throughlaser head 68.

From aperture 77 the beam proceeds to folding mirror 78, and continuesthrough cylindrical lens 81' and 81". The cylindrical lenses are used toremove astigmatism and excess ellipticity from the beam. Excessellipticity is determined based on an aspect ratio of the beam that isdefined as the ratio of minimum beam dimension at a focal plane and themaximum beam dimension at the focal plane. An aspect ratio of oneimplies the beam is circular while an aspect ratio of 1.1 or 0.9 impliesthat the width of the beam in one dimension is approximately 10% greaterthan or less than the width in the other dimension. Aspect ratios inexcess of 1.1 or 0.9 are generally considered excessive though in somecircumstances the beams may be useable.

From cylindrical lens 81"the beam proceeds to folding mirror 78. Most ofthe beam then proceeds through beam splitter 94, while a very smallportion (e.g. around 1-4%) is reflected from the beam splitter back todetector 85 where a power measurement may be taken which can then beused in determining the overall power in the beam. The main portion ofthe beam moves through lenses 83' and 83" in the beam focusing module80. After passing through lens 83" the beam direction is reoriented bytwo folding mirrors 78.

The beam then reenters the focusing module and passes through movablelens 83'". The position of lens 83'" is controlled by stepper motor 87,moveable mount 88, and lead screw 89. The motor is computer controlledso that the beam focal plane may be varied depending on the desired beamsize at the surface of the building material.

It is preferred that the focus system be precalibrated so thatadjustment from one beam size to another may be accomplished withoutdelay. In this regard it is preferred that an encoder provide steppermotor position and that the computer contain a table of encoderpositions corresponding to different desired beam sizes. Based on valuesin the look up table, the stepper motor can be commanded to move to anew position based on a difference between present position and desiredposition. Once the new position is reached, if desired, the actual beamdiameter may be checked using a beam profiling system as described inpreviously referenced U.S. Pat. No. 5,058,988. Various alternativeapproaches to setting beam size will be apparent to those of skill inthe art.

The beam then proceeds to folding mirror 78 and out exit window 90 whereafter the beam encounters the scanning mirrors or other opticalcomponents. The beam produced by this laser head is pulsed at a useablefrequency (e.g. 22.5-40 KHz or more). The laser head is preferably watercooled by passing water in through the base plate that supports thecomponents depicted in FIG. 1d. The water preferably enters the platethrough orifice 91 proceeds along a winding flow path and then exits theplate at orifice 92.

A laser power supply may be used to control operation of the laser inseveral ways: (1) it supplies a desired amount of electric power to thelaser diodes 71 to produce a desired optical output (e.g. about 18watts), (2) it controls thermal electric heaters/coolers or otherheaters/coolers to control the temperatures of the laser diodes, the IRlaser, and/or the conversion crystals, (3) it may control the AOMQ-switch, (4) it may control the focusing system, (5) it may be used tocontrol the detector and to interpret signals therefrom. Alternatively,or additionally, the process computer may be used to control one or moreof the above noted elements. The process computer preferably isfunctionally connected to the laser power supply so that it may furthercontrol laser operation.

A preferred laser head, IR module, and power supply is sold bySpectra-Physics of Mountain View, Calif., as part numberJ30E-BL10-355Q-11 or J30E-BL6-355Q-11.

The water passing through the base plate is also preferably used to coolthe IR laser diodes 71. It is preferred that the water pass through thebase plate prior to passing on to the laser diodes 71. The water may berecirculated through an enclosed cooling system or other recirculatingor non-recirculating system. Various alternatives to water cooling arepossible and will be apparent to those of skill in the art.

Preferred control and data manipulation systems and software aredescribed in a number of the patents referenced above, including U.S.Pat. Nos. 5,184,307; 5,321,622; and 5,597,520.

Referring now to FIGS. 1a and 1b, a preferred recoating device isdescribed in U.S. Pat. No. 5,902,537 as referenced above and includesrecoater bar 13, regulated vacuum pump 17, and vacuum line 19 connectingthe bar 13 and the pump 17.

Other components of a preferred SLA (not shown) may include a liquidlevel control system, a build chamber, an environmental control systemincluding a temperature control system, safety interlocks, a viewingdevice, and the like.

SLAs on which the instant invention can be utilized are available from3D Systems, Inc. of Valencia, Calif. These SLAs include the SLA-250system using a CW HeCd laser operating at 325 nm, the SLA-3500,SLA-5000, and the SLA-7000 systems using solid state lasers operating at355 nm with a pulse repetition rates of 22.5 KHz, 40 KHz, and 25 KHz,respectively. Preferred building materials are photopolymersmanufactured by CIBA Specialty Chemicals of Los Angeles, Calif., and areavailable from 3D Systems, Inc. These materials include SL 5170, SL5190, and SL 5530HT.

The typical operation of an SLA involves alternating formation ofcoatings of material (i.e. layers of material) and the selectivesolidification of those coatings to form an object from a plurality ofadhered laminae. The process may conceptually be viewed as beginningwith the elevator platform 9 immersed one layer thickness below theupper surface 20 of the photopolymer 5. The coating of photopolymer isselectively exposed to prescribed stimulation (e.g. a beam of UVradiation) which cures the material to a desired depth to form aninitial lamina of the object adhered to the elevator platform. Thisinitial lamina corresponds to an initial cross-section of the object tobe formed or corresponds to an initial cross-section of supports thatmay be used to adhere the object to the platform. After formation ofthis initial lamina, the elevator platform and adhered initial laminaare lowered a net amount of one layer thickness into the material.

Hereinafter, layer thickness and other units of distance may beexpressed in any of three units: (1) inches, (2) milli-inches (i.e.mils), or (3) millimeters . As the material is typically very viscousand the thickness of each layer is very thin (e.g. 4 mils to 10 mils),the material may not readily form a coating over the last solidifiedlamina (as shown in FIG. 1a). In the case where a coating is not readilyformed, a recoating device may be swept at or somewhat above the surfaceof the building material (e.g. liquid photopolymer) to aid in theformation of a fresh coating. The coating formation process may involvethe sweeping of the recoating bar one or more times at a desiredvelocity.

After formation of this coating, the second layer is solidified by asecond exposure of the material to prescribed stimulation according todata representing a second cross-section of the object. This process ofcoating formation and solidification is repeated over and over againuntil the object is formed from a plurality of adhered layers (21, 23,25, 27, 29, 31, and 33).

In some building techniques, incomplete solidification of some or allobject cross-sections may occur. Alternatively, in some processes anobject lamina associated with a given layer (i.e. a lamina whoselocation should be positioned, relative to the rest of the object, atthe level corresponding to that layer of material) may not be exposed ormay be only partially exposed in association with that layer (i.e. whenthat layer is located at the surface of the liquid). Instead, thatlamina may be formed in whole or in part in association with asubsequently formed layer wherein the exposure applied to thissubsequent layer is such as to cause material transformation to such anextent as to cause solidification in the material at the level of theassociated cross-section. In other words, the layer which is associatedwith a given lamina may not be the layer in association with which thelamina will be solidified. It may be said that the layer in associationwith which a lamina or portion of a lamina is formed, is that layerwhich is located at the surface of material at the time the lamina issolidified. The layer with which a lamina is associated, is that layerwhich corresponds to the dimensionally correct location of the laminarelative to the rest of the object.

FIG. 2a depicts a side view of an object 41 to be producedstereolithographically. In terms of forming horizontal layers, thisfigure depicts the vertical axis (Z) and one of the horizontal axes (X).This object will be used to illustrate some aspects of a preferredembodiment and alternative embodiment of the instant invention. Thisobject includes two horizontal (i.e. flat) down-facing features: one atthe bottom 43 of the object and the other at the upper edge 45 of thehole 47 through the middle of the object. Similarly, this objectincludes two horizontal (i.e. flat) up-facing features: one at the top49 of the object and the other at the lower edge 51 of the hole 47through the middle of the object. This object includes two verticalwalls 53 and 55 located on either side of hole 47. This object alsoincludes two non-horizontal (sometimes called, near flat) up-facingregions 57 and 59 located on either side of the object and twonon-horizontal down-facing regions 61 and 63 located on either side ofthe object.

FIG. 2b illustrates the object as it might be formed with a desiredresolution using stereolithography wherein the MSD and MRD (discussed inU.S. Pat. Nos. 5,597,520 and U.S. Pat. No. 5,902,538) of the materialare both less than or equal to the desired layer thickness (i.e.resolution). In this example, the thickness 220 of each layer is thesame. As indicated, the object is formed from 16 adhered laminae 101-116and 16 associated layers of material 201-216. As layers are typicallysolidified from their upper surface downward, it is typical to associatecross-sectional data, lamina and layer designations with the upperextent of their positions. To ensure adhesion between laminae, at leastportions of each lamina are typically provided with a quantity ofexposure that yields a cure depth of more than one layer thickness. Insome circumstances use of cure depths greater than one layer thicknessmay not be necessary to attain adhesion. To optimize accuracy it istypical to manipulate the object data to account for an MSD greater thanone layer thickness or to limit exposure of down-facing regions so thatthey are not cured to a depth of more than one layer thickness.

A comparison of FIG. 2a and 2b indicates that the object as reproducedin this example is oversized relative to its original design. Verticaland Horizontal features are positioned correctly; but those featureswhich are sloped or near flat (neither horizontal nor vertical), havesolidified layers whose minimal extent touches the envelope of theoriginal design and whose maximum extent protrudes beyond the originaldesign. Further discussion of data association, exposure, and sizingissues can be found in U.S. Pat. Nos. 5,184,307 and 5,321,622 as well asa number of other patents referenced above.

FIG. 2c depicts the object as produced in FIG. 2b but with variousregions of the object and object laminae distinguished. In oneclassification scheme (as described in U.S. Pat. No. 5,321,622) eachlamina of the object can be made up of one, two or three differentregions: (1) down-facing regions; (2) up-facing regions, and (3)continuing regions (i.e. regions that are neither down-facing norup-facing). In this scheme, the following eight vector types might beutilized though others may be defined and used:

Down-facing boundaries--Boundaries that surround down-facing regions ofthe object.

Up-facing boundaries--Boundaries that surround up-facing regions of theobject.

Continuing boundaries--Boundaries that surround regions of the objectthat are neither up-facing nor down-facing

Down-facing Hatch--Lines of exposure that are positioned within thedown-facing boundaries. These lines may be closely or widely spaced fromone another and they may extend in one or more directions.

Up-facing Hatch--Lines of exposure that are positioned within theup-facing boundaries. These lines may be closely or widely spaced fromone another and they may extend in one or more directions.

Continuing Hatch--Lines of exposure that are positioned withincontinuing boundaries. These lines may be closely or widely spaced fromone another and they may extend in one or more directions.

Down-facing Skin/Fill--Lines of exposure which are positioned within thedown-facing boundaries and closely spaced so as to form a continuousregion of solidified material.

Up-facing Skin/Fill--Lines of exposure which are positioned within theup-facing boundaries and closely spaced so as to form a continuousregion of solidified material.

Taken together, the down-facing boundaries, down-facing hatch anddown-facing fill define the down-facing regions of the object. Theup-facing boundaries, up-facing hatch, and up-facing fill, define theup-facing regions of the object. The continuing boundaries andcontinuing hatch define the continuing regions of the object. Asdown-facing regions have nothing below them to which adhesion isdesirably achieved (other than possibly supports), the quantity ofexposure applied to these regions typically does not include an extraamount to cause adhesion to a lower lamina though extra exposure mightbe given to appropriately deal with any MSD issues that exist. Asup-facing and continuing regions have solidified material located belowthem, the quantity of exposure applied to these regions typicallyincludes an extra amount to ensure adhesion to a lower lamina.

Table 2 outlines the different regions found on each lamina for FIG. 2c.

                  TABLE 2                                                         ______________________________________                                        Object Regions Existing on Each Lamina of FIG. 2c                             Lamina & Down-Facing   Up-Facing                                                                              Continuing                                    Layer    Region(s)     Region(s)                                                                              Region(s)                                     ______________________________________                                        101,201  231                                                                  102,202  232                    272                                           103,203  233                    273                                           104,204  234                    274                                           105,205  235                    275                                           106,206  236           256      276                                           107,207  237                    277                                           108,208  238                    278                                           109,209                259      279                                           110,210                260      280                                           111,211  241           261      281                                           112,212                262      282                                           113,213                263      283                                           114,214                264      284                                           115,215                265      285                                           116,216                266                                                    ______________________________________                                    

Other schemes for region identification and vector type creation aredescribed in various patents and applications referenced above,including U.S. Pat. Nos. 5,184,307; 5,209,878; 5,238,639; 5,597,520;5,902,538; 5,943,235 and, Application Ser. No. 08/855,125. Some schemesmight involves the use of fewer designations such as: (1) defining onlyoutward facing regions and continuing regions where down-facing andup-facing regions are combined to form the outward facing regions; (2)combining all fill types into a single designation; or (3) combiningup-facing and continuing hatch into a single designation or even allthree hatch types into a single designations. Other schemes mightinvolve the use of more designations such as dividing one or both of theup-facing and down-facing regions into flat regions and near-flatregions.

Other region identifications might involve the identification of whichportions of boundaries regions associated with each lamina are outwardfacing and/or interior to the lamina. Outward facing boundary regionsare associated with the Initial Cross-Section Boundaries (ICSB). TheICSB may be considered the cross-sectional boundary regions existingprior to the cross-sections into the various desired regions.

ICSBs are described in U.S. Pat. Nos. 5,321,622 and 5,597,520. Interiorboundaries are bounded on both sides by object portions of the laminawhereas outward boundaries are bounded on one side by an object portionof the lamina and on the other side by a non-object portion of thelamina.

We next turn our attention to specific preferred embodiments of theinstant invention which will be described in view of the preliminaryinformation and background provided above. The headers associated withthe following embodiments are intended to aid reading this disclosurebut are not intended to isolate or limit the applicability of theteachings herein to those individual embodiments in connection withwhich explicit disclosure is made.

First Preferred Embodiment:

FIG. 3 depicts a flowchart for a first preferred embodiment. Thisembodiment calls for reducing the production of synergistic stimulationduring periods of time when the beam is not needed. In this embodimentit is preferred that the beam is not merely inhibited from reaching thesurface of the building material, but that the production of thestimulation is reduced, and more preferably ceased, during theseperiods.

Element 300 indicates that the building material is exposed with a beamof prescribed stimulation. Element 302 indicates that an analysis ismade to determine whether or not the next expose will occur within atime T₁. If exposure is to occur within time T₁, the process loops backto element 300 so that exposure may continue. If exposure is not tooccur within time T₁, the beam power is inhibited from reaching afrequency conversion crystal as indicated by element 304. Element 306indicates that an analysis is performed to determine whether or not anext exposure will occur in a time T₂. If exposure is not to occurwithin the time T₂, the process continues to loop through element 306.If it is determined that exposure should occur within a time T₂, poweris reapplied to the frequency conversion crystal (element 308) so thatprescribed stimulation production is reinitiated and exposure may againoccur according to element 300.

An advantage to this technique is in extending the effective life of thelaser system. In this context the term "effective life" refers to thenumber of hours of object formation that may be obtained from the laserbetween repairs. When a frequency converted laser is used in producingultraviolet radiation, damage to the exiting surface of the frequencyconversion crystal that is responsible for UV radiation production hasbeen observed. This damage has been responsible for significantlyshortening laser life. As the extent of damage to the UV radiationproducing crystal appears to be directly related to the power producedby the crystal and the time of operation, the present embodimentlengthens the effective life of the laser by reducing the power exitingthe crystal. A preferred laser for use in this embodiment is the laserillustrated in FIGS. 1c and 1d. As indicated, an AOM (i.e. acousto-opticmodulator) 72 is located between IR laser head 70 and two frequencyconversion crystals 74 and 76. The AOM is controlled by the systemcontrol computer (e.g. process computer) to inhibit the power fromreaching the frequency conversion crystals when it is not needed forexposing the building material 5 at surface 20 or for some otherpurpose. As it is not uncommon for recoating time, and other periods ofnon-exposure to exceed over 50% of the actual time for forming anobject, it is possible for this technique to double or even furtherextend laser life.

The result of this technique is illustrated in FIG. 4 where a plot oflaser output power (output of prescribed stimulation is shown as afunction of time). In this plot, the lapse of time covers the exposureof three layers and the formation of two layers.

Several layer formation events are depicted in the Figure: (1) PB=Beamprofiling and analysis, (2) Expose=Exposure of a layer to form a lamina,(3) Pd=Predip delay, (4) Coat=time to form a layer over a previouslyformed lamina which is typically the time to sweep a recoating deviceover the previously formed lamina, and (5) Z-wait=delay time aftersweeping before exposure begins.

As indicated, during periods of exposure the prescribed stimulation isproduced at desired levels for object production. Also as indicated,during non-exposure times the quantity of stimulation is drasticallyreduced. During non-exposure time extending more than a few seconds, itis preferred that prescribed stimulation production be reduced to under50% of its exposure level, more preferably under 75%, even morepreferably under 90%, and most preferably it is completely inhibited.

Besides the periods of inhibition noted in FIG. 4, other periods mayexist when inhibition can occur. One such time is known as interhatchdelay and is described in U.S. patent application Ser. No. 09/246,504.Inhibition or reduction may occur during all of these periods, a portionof each period or even just a portion of one of these periods or duringsome other period.

Various ways may be used to reduce the laser power. The faster theability to reduce the power and then to reestablish the power, the moreeffective the technique of this embodiment can be. As noted above, anacousto-optic modulator may be used to vary the power reaching thefrequency conversion crystal(s). As the AOM can be used to completelyinhibit production or to vary the power anywhere between 0% and 100%within a fraction of a second, it is a preferred device for thisembodiment as well as for other embodiments to be discussed hereafter.

Other techniques for controlling laser power include: (1) a mechanismfor variably supplying electric power to a laser diode source thatsupplies pumping energy to the laser source, (2) a mechanism forvariably controlling operation of a Q-switch in the laser source, (3) anelectro-optic modulator, (4) a mechanism for variably controlling apulse repetition rate of the power in the beam, (5) a mechanism forcontrolling the temperature of a laser diode source that suppliespumping energy to the laser source, (6) a mechanism for controlling atemperature of a frequency conversion crystal through which the beamfrom the laser source passes, and (7) a computer controlled shutter.

The time period T₁ may be based on several factors. For example, thesefactors may include (1) time to attenuate or inhibit the beam, and (2)time to reactivate the beam and stabilize it. The time period T₂ may bebased on several factors as well including (2) above and the periodbetween recheck reevaluations. In an alternative, the decision to turnon the beam may be based on the lapse of a count down clock as opposedto looping through a comparison routine.

Second Preferred Embodiment:

This embodiment provides a technique for effectively controlling vectorexposure especially when high scan speeds are utilized. This techniquelinks selected exposure vectors (i.e. vectors which are intended toexpose building material) with one or more non-exposure vectors (i.e.vectors which are used to redirect the beam scanning direction and speedwithout significantly exposing the building material) so it is ensuredthat at the beginning of an exposure vector the scanning speed anddirection of movement are appropriate for the vector to be traced.Likewise, at the end of an exposure vector it is ensured that thescanning speed remains appropriate for the vector.

A flowchart representing an implementation of this embodiment isprovided as FIG. 5. FIG. 5 starts off with Element 400 which sets avariable "i" equal to one. This variable provides a designation for eachexposure vector that is to be drawn. The next consecutive exposurevector is designated "i+1".

Element 402 calls for supplying data representing a first exposurevector, EV_(I), and a second exposure vectors, EV_(i+1). Some parametersfor each vector include: (1) beginning X position for each vector,Xi_(b), X(i+1)_(b) ; (2) beginning Y position for each vector, Yi_(b),Y(i+1)_(b) ; (3) ending X position for each vector, Xi_(e), X(i+1)_(e) ;(4) ending Y position for each vector, Yi_(e), Y(i+1)_(e) ; (5) Xcomponent of scanning speed for each vector, SX_(i) and SX_(i+1) ; and(6) ) Y component of scanning speed for each vector, SY_(i) andSY_(i+1).

Element 404 calls for supplying values for four global controlparameters: (1) HSBorder: Maximum per axis drawing speed for bordersthat do not require ramps=N1; (2) HSRamp: Speed change attainable whenapplying maximum acceleration=N2;

(3) HSRest: Speed at which change of direction transitions are allowedto occur=N3; and (4) FF: time period for applying feed forward commandsto the ends of some vectors=N4. Some preferred values for theseparameters include HSBorder=70 ips (i.e. inches/second), HSRamp=25ips/tick, HSRest=70 ips, and FF=4 ticks. In a preferred system 1 tick=15microseconds.

Element 406 calls for determining the difference in speed along each ofthe X and Y axes between the first and second vectors. This informationalong with the global parameters 406 are taken as input to Element 408.

Element 408 calls for an analysis of whether or not either of ΔSX or ΔSYis greater than N1. If this condition is met, it means that a transitionbetween the two vectors cannot occur without the introduction of two ormore non-exposure vectors. If the response is "yes", the processproceeds to element 410 where the process of generating non-exposurevectors begin. Alternatively, if the response is "no", the processproceeds to element 424 where another inquiry is made.

Element 410 calls for applying feed forward acceleration control at theend of the "i"th exposure vector EV_(i) for a period of N4, blocking thebeam when the end of the "i"th exposure vector EV_(i) is reached, andinserting a first ramp vector RV1_(i) parallel to the "i"th exposurevector EV_(i) at the end of EV_(i). Feed forward is the concept ofapplying acceleration commands prior to the time that a change indirection or speed is to occur. The amount of acceleration force toapply and the time over which to apply it may be empirically determinedbased on optimizing the positioning and scanning speed at the end of thefirst vector and at the beginning of the second vector. It may bepreferred to error on the side of rounding corners than overshootingthem. This optimization may be based on minimizing the overall error inposition and/or scanning speed that results when the transition is made.In a present preferred embodiment, scanning mirror commands arepreferably updated every 15 microseconds. Each 15 microsecond period isconsidered one "tick". In a preferred system, the maximum accelerationhas been set to approximately 25 inches/sec/tick. In this system, it hasbeen empirically determined that use of a feed forward period of 4 ticksgives good results. Of course other values of N4 may be used inspecifying the feed forward period depending on the system conditionsand any desired positioning and speed tolerance criteria.

Element 412 calls for setting the time and/or length of the first rampvector RV1 to a minimum amount necessary to allow both the X-scanner andthe Y-scanner to reach a desired scanning speed when a desired maximumlevel of acceleration N2 is applied. The scanning time for the firstramp vector may be expressed mathematically as the greater of:

    (SX.sub.i -N3-N4*N2)/N2

    or

    (SY.sub.i -N3-N4*N2)/N2.

The length of the ramp vector may be determined from the derived timingand the acceleration value N2 being used.

Element 414 calls for Creating a transition vector TV_(i) starting atend of the first ramp vector RV1_(i) and extending in the same directionas the first ramp vector for a length of time equal to a normal feedforward amount N4. In this embodiment the entire length of the vectorreceives feed forward acceleration commands. The feed forward commandsaccelerate each scanner appropriately to transition to a jump vectorthat will be created in element 420. As such, at this point in theprocess the feed forward criteria can not be specifically set.

Element 416 calls for Inserting a second ramp vector RV2_(i) parallel tothe next exposure vector EV₁₊₁ so that the second ramp vector RV2₁ endsat the beginning of EV_(i+1). The X and Y components of scanning speedat the end of the second ramp vector are equal to the desired values forthe next exposure vector.

Element 418 calls for setting the time/length of the second ramp vectorRV2_(i). The time/length is set to an amount greater than or equal tothe minimum necessary to transition from a scanning speed N3 of the jumpvector to the scanning speed of the next exposure vector. The timeperiod for scanning the second ramp vector may be specified to be theequal to or greater than the larger of

    (SX.sub.i+1 -N3)/N2,

    or

    (SY.sub.i+1 -N3)/N2.

The length of the ramp vector may be determined from the derived timingand the acceleration value N2 being used.

Element 420 calls for inserting a jump vector JV_(i) from the end of thefirst ramp vector RV1_(i) to the beginning of the second ramp vectorRV2₁. Feed forward acceleration commands are applied over the last N4ticks of the jump vector JV_(i). At the end of the jump vector (i.e.beginning of the next exposure vector), the propagation of the beam isuninhibited so that it is allowed to progress through the optical systemto the building material.

Element 422 calls for controlling the scanning mirrors according to theexposure vectors and any generated non-exposure vectors.

Element 424 is reached by conclusion in element 408 that the change inboth the X and Y scanning speed components is less than an acceptableamount set by the HSBorder variable. Element 424 calls for an analysisof whether the ending point of the "i"th exposure vector EV_(i) iscoincident with the beginning point of the (i+1)th exposure vectorEV_(i+1).

If the ending points are equivalent, then the process proceeds toElement 422. By failing the criterion of element 408 and passing thecriterion of element 424, it may be concluded that a transition betweenthe "i"th exposure vector and the (i+1)th exposure vector may be madewith sufficient accuracy using only feed forward commands that will beapplied at the end of the "i"th exposure vector.

If the criterion of element 424 is not met, the process proceeds toelement 426 wherein a transition vector JV_(i) is inserted between the"i"th and (i+1)th exposure vectors. This transition vector is used tobridge the gap between the two vectors.

Additional non-exposure vectors are not typically needed as it ispossible to achieve the desired changes in direction and speed based onuse of feed forward acceleration commands at the end of the "i"thtransition vector and the end of jump vector JV_(i).

Element 428 inquiries as to whether EV_(i) is the last vector to beformed. If it is not, variable "i" is incremented by one (element 432)and the process loops back through elements 402 to 428. If the "i"thexposure vector is the last vector, the process proceeds to element 430where the beam is inhibited and the process ended.

Application of the procedure outlined in FIG. 5 is illustrated with theaid of FIGS. 6 and 7. FIG. 6 depicts a top view of a set of vectors foruse in forming a hypothetical lamina. These vectors represent across-section of the object to be formed and are laid out in the X-Yplane. These vectors include a set of four boundary vectors 440, 442,444, and 446. They also include a set of vectors 448, 450, and 452internal to the boundary and parallel to the Y-axis (e.g. Y-hatch orY-fill vectors). These cross-sectional vectors also include a set ofvectors 454, 456, and 458 internal to the boundary and parallel to theY-axis (e.g. Y-hatch or Y-fill vectors). Each of these groupings ofvectors may utilize different quantities of exposure, may have differentposition tolerance criteria, and may be formed with different beamsizes. As such, the beam power used with each of these sets may bedifferent.

The transition between two of the boundary vectors 444 and 446 isdepicted in

FIG. 7. Even though the two boundary vectors have a coincident point,the combination of their respective scanning speeds and angle result ina transition which cannot be made with sufficient accuracy without usinga series of non-exposure vectors. As such, FIG. 7 depicts a first rampvector 460 beginning at the end of exposure vector 444, extending in adirection parallel to that of vector 444, and having a length necessaryto transition the scanning speed of 444 down to a desired amount (i.e.HSRest). A transition vector begins at the end of ramp vector 460,extending in a direction parallel to that of the ramp vector, and havinga length equal to the desired Feed forward amount (e.g. 4 ticks). Thetransition vector is followed by a jump vector that extends to thebeginning of a second ramp vector 466. Feed forward commands aresupplied at the end of jump vector 464 to make the transition to thedirection of the second ramp vector without necessarily changing the netscanning speed. The second ramp vector 466 connects the jump vector 464to the next exposure vector 446. The length of the ramp vector issufficient to allow the scanning speed to attain the desired value ofthe next exposure vector.

FIG. 8 depicts three plots of values for scanning variables (i.e. IRpower production, UV power reaching the vat, and scanning speed) versusthe two exposure vectors bridged by the non-exposure vectors of FIG. 7.As indicated in the lower portion of the figure, the IR power productionof the laser preferably remains the same. As indicated in the middleportion of the figure, it is preferred that UV power reaches the vatonly during scanning of the two exposure vectors 444 and 446. It ispreferred that UV power production cease during the scanning of thenon-exposure vectors. With an AOM acting as the beam inhibitor, it ispossible to shut down the beam and revive it within a few microseconds.The upper portion of the figure provides a plot of the net scanningspeed resulting from the speed of scanning of the two substantiallyorthogonal is mirror scanners. As indicated, the exposure vector 444 isscanned with a large speed 470, the ramp vector 460 ramps the speed downto a desired lower amount, the transition vector 462 maintains the samenet speed, the second ramp vector increases the scanning speed to adesired amount 472 for exposure vector 446.

Many alternatives to this embodiment exist and will be apparent to thoseof skill in the art. Examples of such alternatives include performingthe coincidence check of element 424 prior to performing the speeddifference check of Element 408. A jump vector may be initiated at theend of first exposure vector EV_(i) without ramping the scanning speed.Different quantities of feed forward may be used ranging from 0 ticksup.

Different values for the global control parameters may be used.Different global control parameters may be used. The parameter valuesmay be different for different elements of the process. For example, adifferent amount of feed forward may be applied to different vectortypes.

Third Preferred Embodiment

The third preferred embodiment provides a technique for adjusting thepower of the prescribed stimulation. Element 500 calls for setting aprocess control variable "i" equal to one. Element 502 calls fordetermining a desired laser power DLP based on desired exposure for eachof the vectors making up an "i"th vector set VS(i). The vector set maybe made up of various vectors. For example, VS may include all vectorsof a single type on a given cross-section. VS may include all vectors ofall types on a single cross-section or on a plurality of cross-sections.The individual vectors in VS may be given different exposures but acommon laser power is used in drawing with the vectors.

Element 504 calls for determining actual laser power (ALP) by temporallydirecting the beam at a sensor. It is preferred that this sensor be afull area detector or a point or slot detector from which the full beampower may be measured. It is preferred that this sensor be located alongthe optical path beyond the scanning mirrors so that the scanningmirrors may be used to direct the beam onto the sensor at desired timesand then to direct the beam onto the surface of the building material.

Element 506 calls for determining the difference between the actualpower and the desired power,

    ALP-DLP=ΔLP

Element 508 calls for determining whether the difference in laser poweris within a desired tolerance band εLP,

    ALP<δLP

If a positive result is issued by the analysis of element 508, theprocess proceeds to Element 510 which calls for the use of the beam toexpose VS (i) as no change in laser power is necessary. Element 512calls for application of a correction factor to the power based on thedifference in power ΔLP. The process then proceeds to step 514 whichcalls for exposing VS(i) with the corrected beam.

The process then continues from either step 510 or 514, where an inquiryis made as to whether or not the VS(i) is the last vector set. If so,element 520 indicates that the process is complete. If not, theprocedure moves to element 518 where "i" is incremented by one and theprocess loops back to element 500.

Various alternatives to this embodiment are possible. For example,element 512 may involve the correction of beam power based on a knownsetting to obtain a desired power instead of basing the correction on adifference in power. Element 512 may derive new parameter settings froma table correlating parameters setting with either a change in beampower or to absolute value of beam power. Element 512 may use anadjustment and feedback loop in combination with power sensing to setthe laser power to a new desired level either alone or in combinationwith smart adjustments based on the power differences.

Adjustment of beam power preferably occurs by utilization of aninhibiting device (e.g. and AOM) located between a laser resonator andat least one frequency conversion element used in producing theprescribed stimulation. Alternative, beam power may be adjusted by aninhibiting device located along the optical path beyond the frequencyconversion crystals or even within the laser resonator itself. Insteadof using a sensor onto which the beam is temporarily presented, a sensorsuch as sensor 85 in FIG. 1d may be used in combination with a varietyof power adjustment devices (e.g. those noted in conjunction with thefirst preferred embodiment), other than the AOM.

To optimize object formation it is preferred that the vector set be assmall as possible. In particular, it is preferred that the vector setinclude less than all vectors in association with a particularcross-section. In other words, it is preferred that more than one vectorset exist for each cross-section.

Vector sets may be based on the vector types noted previously for eachbeam size that will use those vectors. It is preferred that the poweradjustment be achieved in less than 1 second, more preferably less than0.5 seconds, and most preferably in less than 0.1 seconds. The toleranceon laser power δLP may be as small as a few mW or as large as 10% of thedesired beam power depending on the exact criteria being considered.

The Fourth Embodiment

This embodiment provides a technique for changing laser power based onan estimation of whether or not the change will produce a desiredminimum saving in exposure time. Instead of basing changes in laserpower strictly on whether or not the power level does not match adesired power level. The value of changing power is ascertained bycomparing the difference in scanning time to a value based parameter. Ifthe value of changing power is less than that required by the valuebased parameter, the beam power will remain unchanged.

As an example, drawing time for scanning VS(i) at first power may take afirst period of time, while drawing time at a second higher power maytake a second period of time. If the difference between the first andsecond times does not exceed the time to switch the power, or otherwisemeet a specified value based parameter, then scanning will optimally beperformed using the first beam power. An analogous procedure may be usedfor determining whether to switch between various beam sizes.

Element 600 of FIG. 10 calls for setting a process variable "i" equal toone. Element 602 calls for providing an "i"th vector set VS(i) whereeach vector in the set will be exposed with a beam having a single beampower.

Element 604 calls for obtaining a desired exposure for selected vectortypes, in the vector set VS(i). Element 606 calls for obtaining amaximum desired scanning speed for at least one type of vector in VS(i).Element 608 calls for determining the highest usable laser power HLP foruse in exposing at least one vector type in VS(i). The selected vectortype or types should be those for which an upper speed on scanning mustnot be exceeded.

Element 610 calls for providing an actual, or present, laser power ALP.Element 612 calls for determining a difference between the actual laserpower and the highest useable power. This may be expressed as,

    ALP-HLP=ΔLP

Element 614 inquires as to whether or not the difference in laser poweris greater than zero plus a tolerance a laser power tolerance value.This may be expressed as

    ALP≧0+δLP?

If the response to the inquiry of element 614 is "yes," the processproceeds to Element 616 where the laser power is lowered from the ALP toHLP. Once the laser power is reset the process exposes the VS(I) usingthe HLP (Element 618)

If the response to the inquiry of element 614 was "no," the processmoves forward to element 620 and 622. Element 620 calls for deriving theexpose time, ET_(H) (I), for the full set of vectors in VS(I) using thehighest usable laser power HLP. Element 622 calls for deriving theexpose time, ET_(A) (I), for the full set of vectors in VS(I) using theactual laser power ALP.

Element 624 calls for determining the difference between the exposuretime when using the actual laser power and the exposure time when usingthe highest possible laser power. This may be expressed as

    ET.sub.A (I)-ET.sub.H (I)=ΔET

Element 626 inquires as to whether or not the difference in exposuretime is above a preset value. The preset value provides an indication ofhow much time must be saved in order to warrant a changing the laserpower. This inquiry may be expressed as,

    Is ΔET>δET?

If the inquiry produces a negative response, exposure occurs using theactual laser power (Element 628). If the inquiry produces a positiveresponse, the laser power is increased to the highest useable power(Element 630). Whereafter Element 632 calls for exposing the vector setVS(i) using the highest usable laser power HLP.

Element 634 inquires as to whether the "i" th vector set VS(I) is thelast vector set. If an affirmative response is obtained, the processproceeds to element 636 and determinates. If a negative response isobtained, the process proceeds to element 638 where the variable "i" isincremented by one, after which the process loops back to Element 602,where elements 602-634 are repeated until all the vector sets have beenprocessed.

Various other alternatives and modifications to this fourth embodimentare possible. For example, the derivation of exposure time may be basedon an estimate or on an exact calculation. The preset value δET may be aconstant or a variable. It may take on one value if the change in poweris to cause a dead time in exposure or it may be zero if the change inpower has no impact on build time because the change will occur during anon-drawing period anyway. Some alternatives have been discussed hereinabove while others will be apparent to those of skill in the art.

The Fifth Embodiment

A fifth embodiment of the invention provides another technique forsetting beam power based on consideration of a number of parameters.This embodiment uses a beam consisting of a series of pulses with apulse repetition rate and a beam diameter (the diameter being thecross-sectional dimension of the beam at the working surface of thebuilding material).

In this embodiment a system user specifies a maximum draw speed by meansof a graphical user interface. The maximum draw speed is specified forselected vectors. The selected vectors are those whose scan speeds areconsidered critical to the build process. Alternatively, the vectors forwhich maximum scan speed are specified may be those for which exposuresare known to control the process based on their cure depths and thelike, such that once they are specified, the specification of maximumspeed for the other vector types would not change the process. Based on,inter alia, known material properties, desired cure depths, and maybebeam profile information, the beam power required to produce the maximumvelocity is calculated for each of the vector types. For example, thevector type for which maximum scan speeds are specified may be one typeof boundary and one type of hatch, alternatively boundary only or hatchonly.

A top scanning speed is derived for each vector type. The top speed isbased on the laser beam diameter, pulse repetition rate and an overlapcriteria specified for each vector type. The overlap criterion specifieshow close two consecutive pulses must be so that sufficient overlap isobtained. This overlap is usually considered in terms of percentage ofbeam diameter. A sample equation for top speed is,

    Top Speed=Q*B*(1-OL)

Where Q is the pulse repetition rate in Hz, B is the beam diameter atthe working surface in inches or mm, and OL is the minimum overlapcriteria. The result of the computation is scanning speed ininches/second or mm/second. Overlap criteria may be empiricallydetermined by building test objects with different overlap amounts anddetermining which overlap amounts produce objects with sufficientintegrity, or other build property or build properties. Minimum overlapamounts on the order of 40%-60% of beam diameter have been found to beeffective.

If a multiple beam diameter system and process is used, the small spotlaser power is set to the lowest of:

(1) power for maximum scanning speed for the boundary as derived fromthe amount entered into the graphical user interface;

(2) power as derived from a top speed calculation based on the smallspot beam size, boundary overlap criteria, and desired cure depths,etc.;

(3) a power for a scanning speed hard limit coded into a database foruse with a small spot boundary based on desired cure depths, etc.;

(4) power for maximum scanning speed for the hatch as derived from theamount entered into the graphical user interface;

(5) power as derived from a top speed calculation based on the smallspot beam size, hatch overlap criteria, desired cure depths, etc.;

(6) a power for a hard limit coded into a database for use with a smallspot hatch based on desired cure depth etc.;

If a single sized beam is used instead of two or more beams, the limitderived by the above process would be used to set the laser power. Asimilar set of comparisons as noted above would be used in setting thelarge spot laser power.

The above process may be carried out based on the comparison noted aboveor on other comparisons providing the same or a similar result. Forexample, the speeds of (1), (2) and (3) may be compared and the lowestof those speeds used in determining the maximum usable laser power forsmall spot boundary. Similarly, the speeds for (4), (5), and (6) may becompared and the lower value used in determining the maximum usablelaser power for small spot hatch. The maximum laser powers for smallspot hatch and boundary may then be compared and the lower valueselected as the maximum usable spot for use with small spot and thevectors in the set of vectors considered. The process may be repeatedfor determination of large spot power settings.

Many alternatives to this procedure exist. For example, the vector typesconsidered in the comparison above need only be those which are includedin the vector set being considered. Maximum laser power may bedetermined for different types of boundaries, hatch, and even fill.Maximum laser power need not be based on a user-identified maximumscanning speed in some circumstances. Maximum laser power need not bebased on an existing hard coded limit. The process is still applicablefor a single vector type included in the vector set.

Various further alternatives and modifications to this embodiment arepossible. Some of these alternatives have been discussed herein abovewhile others will be apparent to those of skill in the art.

Further Alternatives:

Implementation of the methods described herein to form apparatus forforming objects according to the teachings herein can be implemented byprogramming an SLA control computer, or separate data processingcomputer, through software or hard coding. Methods and apparatus in anyembodiment can be modified according to the alternative teachingsexplicitly described in association with one or more of the otherembodiments. Furthermore, the methods and apparatus in these embodimentsand their alternatives can be modified according to various teachings inthe above incorporated patents and applications. It is believed that theteachings herein can be applied to other RP&M technologies.

Though particular embodiments have been described and illustrated andmany alternatives proposed, many additional embodiments and alternativeswill be apparent to those of skill in the art upon review of theteachings herein. As such, these embodiments are not intended to limitthe scope of the invention, but instead to be exemplary in nature.

We claim:
 1. A stereolithographic method of forming a three-dimensionalobject from a plurality of adhered laminae by exposing successive layersof a material to a beam of prescribed stimulation, comprising:providinga source of a beam of prescribed stimulation; forming a layer ofmaterial adjacent to any last formed layer of material in preparationfor forming a subsequent lamina of the object; exposing the material tothe beam of prescribed stimulation to form the subsequent lamina of theobject according to a plurality of exposure vectors representing thesubsequent lamina; and repeating the acts of forming and exposing aplurality of times in order to form the object from a plurality ofadhered laminae, wherein providing a plurality of non-exposure vectorsbetween at least some pairs of successive exposure vectors, wherein thenon-exposure vectors comprise a ramp vector and a jump vector.
 2. Themethod of claim 1 wherein the ramp vector includes a first ramp vectorand a second ramp vector with the jump vector occurring intermediatetherebetween.
 3. The method of claim 2 additionally comprising anon-exposure jump vector positioned intermediate to the first rampvector and the second ramp vector.
 4. The method of claim 2 wherein thefirst ramp vector is oriented in a direction parallel to a firstdirection.
 5. The method of claim 3 wherein the first ramp vector isscanned so as to allow the scanning speed of the beam to change from afirst speed to a speed that is substantially no more than a desiredredirection speed.
 6. The method of claim 5 wherein the time forscanning the first ramp vector is determined based on at least (1) adifference between the first scanning speed and the desired redirectionspeed, and (2) a maximum acceptable acceleration of the scanning system.7. The method of claim 5 wherein the length of the first ramp vector isdetermined based on at least (1) a difference between the first scanningspeed and the desired redirection speed, and (2) a maximum acceptableacceleration of the scanning system.
 8. The method of claim 4 whereinthe second ramp vector is oriented in a direction parallel to a seconddirection.
 9. The method of claim 8 wherein the second ramp vector isscanned so as to allow the scanning speed of the beam to change to asecond speed from an initial speed at the beginning of the second rampvector.
 10. The method of claim 9 wherein the time for scanning thesecond ramp vector is determined based on at least (1) a differencebetween the first scanning speed and the desired redirection speed, and(2) a maximum acceptable acceleration of the scanning system.
 11. Themethod of claim 9 wherein the length of second ramp vector is determinedbased on at least (1) a difference between the first scanning speed andthe desired redirection speed, and (2) a maximum acceptable accelerationof the scanning system.
 12. The method of claim 3 wherein thenon-exposure vector further comprises a transition vector between thefirst ramp vector and the jump vector.
 13. A stereolithographicapparatus for forming a three-dimensional object from a plurality ofadhered laminae by exposing successive layers of a material to a beam ofprescribed stimulation, comprising:a source of a beam of prescribedstimulation; a recoating system to form a layer of material adjacent toany last formed layer of material in preparation for forming asubsequent lamina of the object; a scanning system to expose thematerial to the beam of prescribed stimulation to form the subsequentlamina of the object according to a plurality of exposure vectorsrepresenting the subsequent lamina; and a computer programmed to operatethe recoating system and the scanning system to form thc object from aplurality of adhered laminae, wherein software is programmed or hardwareis configured to provide a plurality of non-exposure vectors between atleast some pairs of successive exposure vectors, wherein thenon-exposure vectors comprise a first ramp vector and a jump vector. 14.The apparatus of claim 13 wherein the software is programmed or thehardware is configured to provide the first ramp vector and a secondramp vector with the jump vector occurring intermediate therebetween.15. The apparatus of claim 13 wherein the software is programmed orhardware is configured to orient the first ramp vector in a directionparallel to a first direction.
 16. The apparatus of claim 15 wherein thesoftware is programmed or hardware is configured to scan the first rampvector so as to allow the scanning speed of the beam to change from afirst speed to a speed that is substantially no more than a desiredredirection speed.
 17. The apparatus of claim 16 wherein the software isprogrammed or hardware is configured to determine a time for scanningthe first ramp vector based on at least (1) a difference between thefirst scanning speed and the desired redirection speed, and (2) amaximum acceptable acceleration of the scanning system.
 18. Theapparatus of claim 16 wherein the software is programmed or hardware isconfigured to set a length of the first ramp vector based on at least(1) a difference between the first scanning speed and the desiredredirection speed, and (2) a maximum acceptable acceleration of thescanning system.
 19. The apparatus of claim 17 wherein the software isprogrammed or hardware is configured to orient a second ramp vector in adirection parallel to a second direction.
 20. The apparatus of claim 19wherein the software is programmed or hardware is configured to scan thesecond ramp vector so as to allow the scanning speed of the beam tochange to a second speed from an initial speed at the beginning of theramp up vector.
 21. The apparatus of claim 20 wherein the software isprogrammed or hardware is configured to determine a time for scanningthe second ramp vector based on at least (1) a difference between thefirst scanning speed and the desired redirection speed, and (2) amaximum acceptable acceleration of the scanning system.
 22. Theapparatus of claim 21 wherein the software is programmed or hardware isconfigured to determine a length of the second ramp vector based on atleast (1) a difference between the first scanning speed and the desiredredirection speed, and (2) a maximum acceptable acceleration of thescanning system.
 23. The apparatus of claim 14 wherein the software isprogrammed or hardware is configured to produce at least one additionalnon-exposing vector comprising a transition vector located at the end ofthe first ramp vector.